Various conventional image forming apparatuses are known, among which electrophotographic type image forming apparatuses are widely used. For such an image forming apparatus, a high productivity (print number per unit time) on various sheets (recording materials) such as thick paper sheets is desired.
In such an electrophotographic type image forming apparatus, a speed-up of the fixing operation of a fixing device (image heating apparatus) is desired to enhance the productivity with the thick paper having a large basis weight. However, in the case of thick paper sheet, a larger amount of heat is deprived of the fixing device during the sheet processing as compared with the case of thin paper sheet, and therefore, the heat quantity required by the fixing operation is larger than in the case of the thin paper sheet. A method is known in which the productivity is lowered for the thick paper (the fixing speed is lowered, or the print number per unit time is decreased).
As a method not lowering the productivity, an external heating type has been proposed in which a heating member is contacted to an outer surface of a fixing roller (rotatable heating member) to maintain an outer surface temperature of the fixing roller at a target temperature. In a proposal of such an external heating type, an external heating belt (endless belt) rotatably stretched around two supporting rollers, by which a contact area with the fixing roller is increased significantly, thus improving the temperature maintaining property (Japanese Laid-open Patent Application 2007-212896).
However, it is practically difficult to assemble and maintain the two supporting rollers with high precision parallelism therebetween, and as a result, if the parallelism is not assured, the external heating belt shifts in a widthwise direction, which leads to a deterioration of the traveling stability of the external heating belt.
It would be considered to control the shifting of the external heating belt by inclining one of the supporting rollers relative to the other supporting roller, but it is difficult to employ such a method for the external heating belt which has to perform the function of heating the fixing roller.
This is because with such a method, a one axial end portion side of the one of the supporting roller is displaced relative to the other end portion side, with which a part of a region of the supporting roller which has to contact the external heating belt may separate from the fixing roller by the displacement. If this occurs, the function of heating the fixing roller is deteriorated with the result of improper fixing.